Method and system of dynamically controlling shaping time of a photon counting energy-sensitive radiation detector to accommodate variations in incident radiation flux levels

ABSTRACT

A method and system of counting and tagging radiation energy received by a radiation detector is presented. The method and system are designed to dynamically control the sampling window or shaping time characteristics of a photon counting detector to accommodate variations of flux experienced by the detector so as to preserve optimum detector performance and prevent saturation during high flux conditions.

BACKGROUND OF THE INVENTION

The present invention relates generally to radiographic imaging and,more particularly, to a method and system of dynamically controllingshaping time of an energy-sensitive radiographic detector, such as a CTdetector, to accommodate broad variations in radiation flux levelsexperienced by the detector. The present invention is particularlyrelated to photon counting and/or energy discriminating radiationdetectors.

Typically, in radiographic systems, an x-ray source emits x-rays towarda subject or object, such as a patient or a piece of luggage.Hereinafter, the terms “subject” and “object” may be interchangeablyused to describe anything capable of being imaged. The x-ray beam, afterbeing attenuated by the subject, impinges upon an array of radiationdetectors. The intensity of the radiation beam received at the detectorarray is typically dependent upon the attenuation of the x-rays throughthe scanned object. Each detector element of the detector array producesa separate signal indicative of the attenuated beam received by eachdetector element. The signals are transmitted to a data processingsystem for analysis and further processing which ultimately produces animage.

In a similar fashion, radiation detectors are employed in emissionimaging systems such as used in nuclear medicine (NM) gamma cameras andPositron Emission Tomography (PET) systems. In these systems, the sourceof radiation is no longer an x-ray source, rather it is aradiopharmaceutical introduced into the body being examined. In thesesystems each detector of the array produces a signal in relation to thelocalized intensity of the radiopharmaceutical concentration in theobject. Similar to conventional x-ray imaging, the strength of theemission signal is also attenuated by the inter-lying body parts. Eachdetector element of the detector array produces a separate signalindicative of the emitted beam received by each detector element. Thesignals are transmitted to a data processing system for analysis andfurther processing which ultimately produces an image.

In most computed tomography (CT) imaging systems, the x-ray source andthe detector array are rotated about a gantry encompassing an imagingvolume around the subject. X-ray sources typically include x-ray tubes,which emit the x-rays as a fan or cone beam from the anode focal point.X-ray detector assemblies typically include a collimator for reducingscattered x-ray photons from reaching the detector, a scintillatoradjacent to the collimator for converting x-rays to light energy, and aphotodiode adjacent to the scintillator for receiving the light energyand producing electrical signals therefrom. Typically, each scintillatorof a scintillator array converts x-rays to light energy. Each photodiodedetects the light energy and generates a corresponding electricalsignal. The outputs of the photodiodes are then transmitted to the dataacquisition system and then to the processing system for imagereconstruction.

Conventional CT imaging systems utilize detectors that convert x-rayphoton energy into current signals that are integrated over a timeperiod, then measured and ultimately digitized. A drawback of suchdetectors is their inability to provide independent data or feedback asto the energy and incident flux rate of photons detected. That is,conventional CT detectors have a scintillator component and photodiodecomponent wherein the scintillator component illuminates upon receptionof x-ray photons and the photodiode detects illumination of thescintillator component and provides an integrated electrical currentsignal as a function of the intensity and energy of incident x-rayphotons. While it is generally recognized that CT imaging would not be aviable diagnostic imaging tool without the advancements achieved withconventional CT detector design, a drawback of these integratingdetectors is their inability to provide energy discriminatory data orotherwise count the number and/or measure the energy of photons actuallyreceived by a given detector element or pixel. Accordingly, recentdetector developments have included the design of an energydiscriminating detector that can provide photon counting and/or energydiscriminating feedback. In this regard, the detector can be caused tooperate in an x-ray counting mode, an energy measurement mode of eachx-ray event, or both.

These energy discriminating detectors are capable of not only x-raycounting, but also providing a measurement of the energy level of eachx-ray detected. While a number of materials may be used in theconstruction of an energy discriminating detector, includingscintillators and photodiodes, direct conversion detectors having anx-ray photoconductor, such as amorphous selenium or cadmium zinctelluride, that directly convert x-ray photons into an electric chargehave been shown to be among the preferred materials. A drawback ofphoton counting detectors, however, is that these types of detectorshave limited count rates and have difficulty covering the broad dynamicranges encompassing very high x-ray photon flux rates typicallyencountered with conventional CT systems. Generally, a CT detectordynamic range of 1,000,000 to one is required to adequately handle thepossible variations in photon flux rates. In the very fast scanners nowavailable, it is not uncommon to encounter x-ray flux rates of over 10⁸photons/mm²/sec when no object is in the scan field, with the samedetection system needing to count only 10's of photons that manage totraverse the center of large objects.

The very high x-ray photon flux rates ultimately lead to detectorsaturation. That is, these detectors typically saturate at relativelylow x-ray flux levels. This saturation can occur at detector locationswherein small subject thickness is interposed between the detector andthe radiographic energy source or x-ray tube. It has been shown thatthese saturated regions correspond to paths of low subject thicknessnear or outside the width of the subject projected onto the detectorarray. In many instances, the subject is more or less cylindrical in theeffect on attenuation of the x-ray flux and subsequent incidentintensity to the detector array. In this case, the saturated regionsrepresent two disjointed regions at extremes of the detector array. Inother less typical, but not rare instances, saturation occurs at otherlocations and in more than two disjointed regions of the detector. Inthe case of a cylindrical subject, the saturation at the edges of thearray can be reduced by the imposition of a bowtie filter between thesubject and the x-ray source. Typically, the filter is constructed tomatch the shape of the subject in such a way as to equalize totalattenuation, filter and subject, across the detector array. The fluxincident to the detector is then relatively uniform across the array anddoes not result in saturation. What can be problematic, however, is thatthe bowtie filter may not be optimum given that a subject population issignificantly less than uniform and not exactly cylindrical in shape norcentrally located in the x-ray beam. In such cases, it is possible forone or more disjointed regions of saturation to occur or conversely toover-filter the x-ray flux and unnecessarily create regions of very lowflux. Low x-ray flux in the projection results in a reduction ininformation content which will ultimately contribute to unwanted noisein the reconstructed image of the subject.

Moreover, a system calibration method common to most CT systems involvesmeasuring detector response with no subject whatsoever in the beam. This“air cal” reading from each detector element is used to normalize andcorrect the preprocessed data that is then used for CT imagereconstruction. Even with ideal bowtie filters, high x-ray flux now inthe central region of the detector array could lead to detectorsaturation during the system calibration phase.

A number of imaging techniques have been proposed to address saturationof any part of the detector. These techniques include maintenance of lowx-ray flux across the width of a detector array, for example, bymodulating tube current or x-ray voltage during scanning. However, thissolution leads to increased scanned time. That is, there is a penaltythat the acquisition time for the image is increased in proportion tothe nominal flux needed to acquire a certain number of x-rays that meetimage quality requirements. Other solutions include the implementationof over-range algorithms that may be used to generate replacement datafor the saturated data. However, these algorithms may imperfectlyreplace the saturated data as well as contribute to the complexity ofthe CT system.

It would therefore be desirable to design an energy discriminating,photon counting CT detector that does not saturate at the x-ray photonflux rates typically found in conventional CT systems.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is a directed method and apparatus for adjustingthe sampling/shaping time characteristics of a radiation detector as afunction of incident photon flux that overcomes the aforementioneddrawbacks.

The present invention includes a method and system of counting andtagging radiating energy received by a radiation detector. The methodand system are designed to dynamically control the sampling time orshaping time characteristics of a photon counting detector toaccommodate large variations of flux experienced by the detector so asto prevent saturation during high flux conditions. Moreover, the presentinvention is designed to control the detector so as to accommodate lowflux rate conditions such that detection efficiency and image quality isnot sacrificed when lower flux is experienced by the detector.

A photon counting (PC) radiographic system includes a radiation energydetector configured to detect radiation energy having a given flux rateand output signals indicative of the detected radiation energy. A shaperunit with a given shaping time is connected to receive the electricalsignals and conditions them to provide electrical pulses indicative ofthe radiation photon energy. A PC channel is connected to receive theelectrical signals and sample the electrical pulse signals of a certainheight or intensity indicative of the photon energy by an adjustablepulse height discriminator or threshold. The PC channel is furtherconfigured to provide a photon count output over a sampling interval.The system also includes a control operationally connected to the PCchannel and configured to automatically adjust the shaping time at leastas a function of the given flux rate. The system also includes a controloperationally connected to the PC channel and configured toautomatically adjust the sensitivity to pulse height or thresholddiscriminator as a function of the given flux rate or shaping time.

A CT system includes a rotatable gantry having a bore centrally disposedtherein and a table movable fore and aft through the bore and configuredto position a subject for CT data acquisition. A radiographic energyprojection source is positioned within the rotatable gantry andconfigured to project radiographic energy toward the subject. The CTsystem further includes a detector assembly disposed within therotatable gantry and configured to detect radiographic energy projectedby the projection source and impinged by the subject. The detectorassembly is defined to include detector elements configured to outputelectrical signals indicative of detected radiographic energy and PCchannels operationally connected to the detector elements and configuredto count the number of photons of the detected radiographic energysignal conditioned according to a variable shaping time. The detectorelements also have shaping time controllers operationally connected tothe PC channels and configured to control the variable shaping times innear real-time based on the photon output count data.

A method of preventing radiographic energy detector saturation includesmonitoring flux of radiographic energy having a number of photonsreceived by a photon counting, radiographic energy detector. Thedetector is designed to sample a photon charge cloud, in the case ofdirect conversion detectors having an x-ray photoconductor, or aphoto-diode current pulse, in the case of scintillator detectors, andcount the number of photons using a given signal pulse shaping time. Themethod further includes comparing a current flux on the radiographicenergy detector to a base flux level corresponding to the given shapingtime and adjusting the given shaping time to correspond to the currentflux based on the comparison. An additional aspect of the presentinvention includes automatic means for modifying the energy thresholdlevels so as to compensate for changing channel shaping times in orderto maintain accurate photon energy information.

Therefore, in accordance with one aspect of the present invention, asingle PC radiographic system includes a radiographic energy detectorconfigured to detect radiographic energy having a given flux rate andoutput electrical signals indicative of the detected radiographicenergy. The system further includes a PC channel connected to receivethe electrical signals and sample the electrical signals in a samplinginterval window and provide a photon count output. A control isoperationally connected to the PC channel and configured toautomatically adjust the sampling interval window at least as a functionof the given flux rate.

According to another aspect, the present invention includes a CT systemhaving a rotatable gantry having a bore centrally disposed therein, atable movable fore and aft through the bore and configured to position asubject for CT data acquisition, a radiographic energy projection sourcepositioned within the rotatable gantry and configured to projectradiographic energy toward the subject, and a detector assembly disposedwithin the rotatable gantry and configured to detect radiographic energyprojected by the projection source and impinged by the subject. Thedetector assembly includes a detector element configured to outputelectrical signals indicative of detected radiographic energy and a PCchannel operationally connected to the detector element and configuredto count a number of photons of the detected radiographic energyaccording to a variable shaping time. The detector assembly furtherincludes a shaping time controller operationally connected to the PCchannel and configured to control the variable shaping time in nearreal-time based on the photon output count data.

In accordance with yet another aspect, a method of preventing radiationenergy detector saturation includes monitoring flux of radiation energyhaving a number of photons received by a photon counting, radiationenergy detector. The detector is designed to sample a photon chargecloud within a given sampling window and count the number of photons.The method further includes comparing a current flux on the radiationenergy detector to a base flux level corresponding to the given samplingwindow and adjusting the given sampling window to correspond to thecurrent flux based on the comparison.

Various other features, objects and advantages of the present inventionwill be made apparent from the following detailed description and thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate one preferred embodiment presently contemplatedfor carrying out the invention.

In the drawings:

FIG. 1 is a pictorial view of a CT imaging system.

FIG. 2 is a block schematic diagram of the system illustrated in FIG. 1.

FIG. 3 is a block schematic diagram of a detector assembly according tothe present invention.

FIG. 4 is a graph illustrating signal amplitude plots for a number ofshaping times for an exemplary PC detector.

FIG. 5 is a pictorial view of a CT system for use with a non-invasivepackage inspection system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The operating environment of the present invention is described withrespect to a four-slice computed tomography (CT) system. However, itwill be appreciated by those skilled in the art that the presentinvention is equally applicable for use with single-slice or othermulti-slice configurations. Moreover, the present invention will bedescribed with respect to the detection and conversion of x-rays.However, one skilled in the art will further appreciate that the presentinvention is equally applicable for the detection and conversion ofother radiation energy sources.

Referring to FIGS. 1 and 2, a computed tomography (CT) imaging system 10is shown as including a gantry 12 representative of a “third generation”CT scanner. Gantry 12 has an x-ray source 14 that projects a beam ofx-rays 16 toward a detector assembly 18 on the opposite side of thegantry 12. Detector assembly 18 is formed by a plurality of detectors 20which together sense the projected x-rays that pass through a medicalpatient 22. Each detector 20 produces an electrical signal thatrepresents not only the intensity of an impinging x-ray beam but is alsocapable of providing photon or x-ray count data and energy level, andhence the attenuated beam as it passes through the patient 22. During ascan to acquire x-ray projection data, gantry 12 and the componentsmounted thereon rotate about a center of rotation 24.

Rotation of gantry 12 and the operation of x-ray source 14 are governedby a control mechanism 26 of CT system 10. Control mechanism 26 includesan x-ray controller 28 that provides power and timing signals to anx-ray source 14 and a gantry motor controller 30 that controls therotational speed and position of gantry 12. A data acquisition system(DAS) 32 in control mechanism 26 reviews data from detectors 20 andconverts the data to digital signals for subsequent processing. An imagereconstructor 34 receives sampled and digitized x-ray data from DAS 32and performs high speed reconstruction. The reconstructed image isapplied as an input to a computer 36 which stores the image in a massstorage device 38.

Computer 36 also receives commands and scanning parameters from anoperator via console 40 that has a keyboard. An associated displayscreen 42 allows the operator to observe the reconstructed image andother data from computer 36. The operator supplied commands andparameters are used by computer 36 to provide control signals andinformation to DAS 32, x-ray controller 28 and gantry motor controller30. In addition, computer 36 operates a table motor controller 44 whichcontrols a motorized table 46 to position patient 22 and gantry 12.Particularly, table 46 moves portions of patient 22 through a gantryopening 48.

The present invention is directed to a radiation detector that may beincorporated with the CT system described above or other radiographicsystems, such as x-ray systems or general purpose radiation detectors.

Generally, high-sensitivity photon counting radiation detectors areconstructed to have a relatively low dynamic range. This is generallyconsidered acceptable for proton counting detector applications sincehigh flux conditions typically do not occur. In CT detector designs, lowflux detector readings through the subject are typically accompanied byareas of high irradiation in air, and/or within the contours of the scansubject requiring CT detectors to have very large dynamic rangeresponses. Moreover, the exact measurement of photons in these high-fluxregions is less critical than that for low-flux areas where each photoncontributes an integral part to the total collected photon statistics.Notwithstanding that the higher flux areas may be of less clinical ordiagnostic value, images reconstructed with over-ranging or saturateddetector channel data can be prone to artifacts. As such, the handlingof high-flux conditions is also important.

The present invention includes an x-ray flux management control designedto prevent saturation of PC x-ray systems having detector channelscharacterized by low dynamic range. Dynamic range of a detector channeldefines the range of x-ray flux levels that the detector channel canhandle to provide meaningful data at the low-flux end and not experienceover-ranging or saturating at the high flux end. Notwithstanding theneed to prevent over-ranging, to provide diagnostically valuable data,the handling of low-flux conditions, which commonly occur during imagingthrough thicker cross-sections and other areas of limited x-raytransmission, is also critical in detector design. As such, the x-rayflux management control described herein is designed to satisfy bothhigh flux and low flux conditions.

Generally, operation of a photon counting detector is characterized by ashaping time curve that is fixed. The shaping time curve defines arelationship or balance between charge integration time (single-eventsignal level) and detector channel recovery time so as to provideacceptable PC count-rates, noise suppression, and energy resolution.Typically, the detector channel is constructed to have a shaping timethat favors low-flux rate conditions. That is, for low-flux rateconditions, which translate to fewer x-ray photons, a longer shapingtime is preferred so that the entire photon charge cloud is integratedand SNR is optimized. There is generally relatively little constraint onthe time necessary to integrate the entire photon cloud. Since thecondition is characterized by low-flux, the detector channel is notlikely to saturate while integrating or otherwise sampling the entirephoton cloud. On the other hand, the low-flux rate favored, fixed timeshaping may be insufficient for high-flux rate conditions. Moreover, ifthe time shaping is fixed to match or correspond to high-flux rateconditions, a negative impact on SNR and energy resolution duringlow-flux rate conditions follows.

Accordingly, the present invention includes a system and method todynamically and automatically control the shaping time of a detectorchannel such that low-flux as well as high-flux rate conditions areoptimally addressed. Referring now to FIG. 3, a block schematic diagramof an x-ray detection system 50 according to the present invention isshown. System 50 includes an PC channel 52 connected to receiveelectrical signals from a detector element 54. Detector 54 isconstructed to detect x-rays 16 projected by an x-ray source andattenuated by a subject, such as a medical patient. It is understoodthat the present invention is applicable with gamma rays and other formsof radiographic energy.

PC channel 52 includes a low-noise/high-speed charge amplifier 56connected to receive the electrical signals from detector element 54.The amplified output of amplifier 56 is then input to a signal shaper 58constructed to extract individual photon events from the electricalsignals. Energy level discriminator 60 is connected to signal shaper 58and is designed to filter photons based on their pulse height energylevel relative to one or more thresholds. To this end, those photonshaving energy levels outside a desired range are excluded from countingand processing for image reconstruction. Minimally, discriminator 60 isdesigned to exclude those photons having an energy level correspondingto noise in the system. It is contemplated that multiple thresholds maybe used to define energy level ranges. Counting element 62 receivesthose photons not filtered out by energy level discriminator 60 and isconstructed to count the number of photons received at the detector andprovide a corresponding output 64. As will be described and in contrastto known PC channels, operation PC channel 52 is governed by a variableshaping time.

PC channel 52 is operationally connected to a control 66 that includes ashaping time controller 68 and, preferably, an energy level controller70. While it is preferred that control 66 include energy levelcontroller 70, it is contemplated that the present invention may becarried out without it. In one embodiment, PC channel 52 includes anactive filter whose operation defines the channel's shaping time. Inthis regard, resistive and capacitive characteristics of the activefilter can be adjusted to manipulate the channel's shaping timeproperties.

Shaping time controller 68 is connected to PC channel 52 and is designedto adjust the shaping time characteristics of PC channel 52 based onphoton count feedback received across feedback loop 72. Moreparticularly, shaping time controller 68 increases the channel's shapingtime when the detector element is exposed to low x-ray flux as measuredby the number of photons counted 64. In contrast, when the x-ray flux onthe detector element 54 increases, the time shaping controller decreasesthe time shaping or sampling window of PC channel 52.

As such, when the detector is experiencing higher x-ray flux, the amountof time the PC channel spends sampling the photon charge cloud isreduced. Accordingly, less precise photon and energy discriminatory datawith respect to the photon charge cloud is determined; however, thechannel recovers at a rate sufficient to avoid over-ranging. In thisregard, as the shaping time or sampling window is caused to decrease,more photons are inspected for data, i.e. counted, while each detectedphoton provides less precise energy discriminatory information. And,under high flux conditions, each individual photon assumes lessimportance and the overall system performance and image quality isminimally impacted by the reduced SNR. On the other hand, when thedetector is experiencing lower x-ray flux, the amount of time the PCchannel spends to sample the photon charge cloud is lengthened whichallows sufficient time to sample the entire photon charge cloud andattain relatively precise photon count and energy discriminatory data.

As referenced above, control 66 includes, in one embodiment, an energylevel controller 70. Since the measured photon signal levels vary withchannel shaping time, automatic energy discriminator energy levelcontroller 70 is coupled to shaping time controller 68 and PC channel 52to adjust or otherwise calibrate the energy level threshold of the PCchannel in response to an adjustment in the shaping time. By performingappropriate channel calibration, photons having an acceptable ordecreased energy level are counted to assure linear energy responseindependent of channel shaping time and count rate.

Referring now to FIG. 4, a number of amplitude plots for several shapingtime curves for an exemplary PC channel are illustrated. Decreasing theshaping time increases the potential count rate but, as shown, decreasesthe signal amplitude and increases noise. Specifically, adjusting thetime shaping defined by curve 74 to that defined by curve 76 increasesthe potential count rate, but causes an inversely related decline incollective signal strength of the counted photons and negatively affectsSNR. A further decrease in shaping time, i.e. curve 76 to curve 78results in a further increase in count rate potential, but withadditional decline in signal strength and SNR.

Referring now to FIG. 5, package/baggage inspection system 80 includes arotatable gantry 82 having an opening 84 therein through which packagesor pieces of baggage may pass. The rotatable gantry 82 houses a highfrequency electromagnetic energy source 86 as well as a detectorassembly 88. A conveyor system 90 is also provided and includes aconveyor belt 92 supported by structure 94 to automatically andcontinuously pass packages or baggage pieces 96 through opening 84 to bescanned. Objects 96 are fed through opening 84 by conveyor belt 92,imaging data is then acquired, and the conveyor belt 92 removes thepackages 96 from opening 84 in a controlled and continuous manner. As aresult, postal inspectors, baggage handlers, and other securitypersonnel may non-invasively inspect the contents of packages 96 forexplosives, knives, guns, contraband, etc.

Therefore, the present invention includes a PC radiographic system. Theradiographic system includes a radiographic energy detector configuredto detect radiographic energy having a given flux rate and outputelectrical signals indicative of the detected radiographic energy. A PCchannel is connected to receive the electrical signals and sample theelectrical signals in a sampling interval. The PC channel is furtherconfigured to provide a photon count output. The system also includes acontrol operationally connected to the PC channel and configured toautomatically adjust the sampling interval at least as a function of thegiven flux rate.

A CT system is presented and includes a rotatable gantry having a borecentrally disposed therein and a table movable fore and aft through thebore and configured to position a subject for CT data acquisition. Aradiographic energy projection source is positioned within the rotatablegantry and configured to project radiographic energy toward the subject.The CT system further includes a detector assembly disposed within therotatable gantry and configured to detect radiographic energy projectedby the projection source and impinged by the subject. The detectorassembly is defined to include detector elements configured to outputelectrical signals indicative of detected radiographic energy and PCchannels operationally connected to the detector elements and configuredto count the number of photons of the detected radiographic energyaccording to a variable shaping time. The detector assembly also haveshaping time controllers operationally connected to the PC channels andconfigured to control the variable shaping times in near real-time basedon the photon output count data.

The present invention further includes a method of preventing radiationdetector saturation. The method includes monitoring flux of radiationenergy having a number of photons received by a photon counting,radiation energy detector. The detector is designed to sample a photoncharge cloud within a given sampling window and count the number ofphotons. The method further includes comparing a current flux on theradiation energy detector to a base flux level corresponding to thegiven sampling window and adjusting the given sampling window tocorrespond to the current flux based on the comparison.

The present invention has been described in terms of the preferredembodiment, and it is recognized that equivalents, alternatives, andmodifications, aside from those expressly stated, are possible andwithin the scope of the appending claims.

1. A photon counting (PC) radiographic system comprising: a radiographicenergy detector configured to detect radiographic energy passing throughan object to be imaged and having a given flux rate and outputelectrical signals indicative of the detected radiographic energy; a PCchannel connected to receive the electrical signals and sample theelectrical signals in a sampling window and provide a photon countoutput; and a control operationally connected to the PC channel andconfigured to automatically adjust the sampling window at least as afunction of the given flux rate.
 2. The system of claim 1 wherein thecontrol is further configured to decrease the sampling window with anincrease in the given flux rate.
 3. The system of claim 1 wherein thecontrol is further configured to increase the sampling window with adecrease in the given flux rate.
 4. The system of claim 1 furthercomprising a feedback loop between the photon count output and thecontrol, and wherein the control is further configured to determine thegiven flux rate based on photon count data received across the feedbackloop.
 5. The system of claim 1 wherein the control is further configuredto adjust an energy level threshold based on an adjustment of thesampling window to accept photons with acceptable energy levels.
 6. Thesystem of claim 1 wherein the radiographic energy detector is configuredto detect radiation energy with a wavelength less than 10 nanometers. 7.The system of claim 6 wherein the radiation energy detector isconfigured to detect x-ray energy.
 8. A CT system comprising: arotatable gantry having a bore centrally disposed therein; a tablemovable fore and aft through the bore and configured to position asubject to be imaged for CT data acquisition; a radiographic energyprojection source positioned within the rotatable gantry and configuredto project radiographic energy toward the subject; and a detectorassembly disposed within the rotatable gantry and configured to detectradiographic energy projected by the projection source and impinged bythe subject, the detector assembly including: a detector elementconfigured to output electrical signals indicative of detectedradiographic energy attenuated by the subject; a PC channeloperationally connected to the detector element and configured to counta number of photons of the detected radiographic energy according to avariable shaping time; and a shaping time controller operationallyconnected to the PC channel and configured to control the variableshaping time in near real-time based on the photon output count data. 9.The CT system of claim 8 wherein the radiographic energy includes x-rayenergy, and wherein the table is designed to position a medical patientwithin the bore.
 10. The CT system of claim 8 wherein the shaping timecontroller is further configured to shorten the variable shaping time asthe number of photons counted increases.
 11. The CT system of claim 8wherein the shaping time controller is further configured to lengthenthe variable shaping time as the number of photons counted decreases.12. The CT system of claim 8 wherein the number of photons counted is afunction of flux of the radiographic energy received by the detectorelement.
 13. The CT system of claim 8 wherein the shaping timecontroller is further configured to control the variable shaping time toprevent saturation of the PC channel.
 14. The CT system of claim 13wherein the variable shaping time defines a balance between chargeintegration time and channel recovery time.
 15. The CT system of claim 8wherein the PC channel includes: a low-noise, high-speed chargeamplifier; a signal shaper operationally connected to the low-noise,high-speed charge amplifier designed to extract individual photonevents; an energy level discriminator operationally connected to thesignal shaper and designed to identify a photon energy for each photonevent; and a photon counting element operationally connected to theenergy level discriminator and designed to count the number of photonsfor a number of photon identified energies.
 16. The CT system of claim 8further comprising an energy level controller operationally connected tothe shaping time controller and designed to accept photon events forcounting having acceptable energy levels.
 17. The CT system of claim 16wherein the energy level controller is further designed to assure linearenergy response independent of the variable shaping time and/or thenumber of photons counted.
 18. A method of preventing radiation energydetector saturation comprising the steps of: monitoring flux ofradiation energy that has passed through an object to be imaged, theradiation energy having a number of photons received by a photoncounting, radiation energy detector, the detector designed to sample aphoton charge cloud within a given sampling window and count the numberof photons; comparing a current flux on the radiation energy detector toa base flux level corresponding to the given sampling window; andadjusting the given sampling window to correspond to the current fluxbased on the comparison.
 19. The method of claim 18 wherein the step ofadjusting includes the step of lengthening the given sampling window ifa level of the current flux is less than the base flux.
 20. The methodof claim 18 wherein the step of adjusting includes the step ofshortening the given sampling window if a level of the current flux ismore than the base flux.
 21. The method of claim 18 wherein the step ofmonitoring includes the step of receiving an indication of the number ofphotons counted by the radiation detector.
 22. The method of claim 18further comprising the step of automatically adjusting an energy levelthreshold in response to an adjustment of the given sampling window. 23.The method of claim 18 further comprising the step of data processingand reconstructing an image of a subject and wherein the image includestissue differentiation.